Lithium is the lightest and most electropositive element, making it well-suited for applications that require high energy density. As such, lithium-ion (Li+) batteries have been successfully employed in a large variety of portable and other electronic devices. However, slow diffusion of Li+ into the anode and the cathode, as well as slow diffusion between the two electrodes, remain the two principal limitations to the rates of charging and discharging for these batteries.
Nanostructured materials have been demonstrated to be useful for Li+ batteries due to their high surface area-to-volume ratio, a property that has been shown to lead to greater reversibility for the lithiation reaction and greater discharge rates. Moreover, fabrication of nanowire arrays of both carbon-based anodes and several common cathode materials has been shown to enhance electrode performance because the reduction in particle size of the electrode materials, while maintaining electrical contact from grain to grain, reduces the distances the Li+ ions must diffuse.
In particular, the charge/discharge rate of a battery is related to the rates of diffusion of Li+ into each electrode and the rate of diffusion between the cathode and the anode. While nanowires have been shown to cycle faster than bulk materials, reducing the distance between cathode and anode battery structures has not been straightforward, and although nanostructured cathodes/anodes have previously been utilized in Li+ batteries, this was done primarily to increase the surface area-to-volume ratio of either the cathode or anode or both, and the Li+ diffusion distance remained, as a consequence, quite large as lithium ions were required to travel large distances between macroscopically separated electrodes.